This invention relates to a soldering method and a gas feeding method for use in mounting electronic parts on a packaged printed circuit board for an electronic circuit by reflow soldering. These methods upgrade the quality of a soldering procedure.
So far the soldering of electronic parts has been performed in an airy atmosphere with the use of a rosin-based cream solder, thus requiring the use of a high-activity flux. The cream solder using the high-activity flux is satisfactory in soldering property, but produces a large amount of residue. Because of this questionable reliability, etc., it needs to be washed with freon or the like. Freon used in the washing step, however, leads to environmental pollution, and its use has been restricted. In an attempt to eliminate the washing step, a cream solder containing a low-activity flux has been developed. A non-washing method which performs soldering using such a cream solder in an inert gas such as nitrogen gas has recently come into practical use.
When a rosin-based cream solder containing a flux with a low solids content is used, reflow soldering is carried out in a nitrogen atmosphere. This soldering procedure in a nitrogen atmosphere enhances soldering property, and generates minimal residue because of a low activity. The low activity, however, may lead to the occurrence of "solder balls," and if many such solder balls develop, a washing step becomes necessary irrespective of minimal residue. A cream solder for a fine pitch comprises small solder particles, and have a higher possibility of producing solder balls.